Abstract Developing HIV envelope (Env) immunogens capable of eliciting antibodies (Abs) effective against a broad array of HIV-1 isolates is a major challenge in HIV vaccine development. Indeed, factors contributing to the generation of broadly reactive Abs with neutralizing and/or nonneutralizing activities are not understood. This proposal will test the concept that induction of Abs against HIV Env is influenced by the presence of anti-Env Abs that form immune complexes (ICs) and that consequently modify epitopes on Env. Specifically, we hypothesize that administration of an IC vaccine made of Env and select anti-Env monoclonal Ab (mAbs) will shape epitope specificity, dominance, and maturation of de novo Ab responses to Env. Our earlier studies have shown that anti-Env mAbs can be used to direct vaccine-induced endogenous Ab responses toward or away from certain Env epitopes; this activity is dictated by the mAb's fine specificity. Immunization of mice with subtype B gp120 in complex with an anti-CD4-binding site (CD4bs) mAb elicited high-affinity Ab responses skewed toward the V3 crown, while an IC made with an anti-V3 mAb blocked induction of V3 crown-specific Abs. In contrast, IC made of gp120 CRF01.AE A244 and anti-V2 mAb 2158 induced a higher Ab response to V1V2 compared to uncomplexed A244 gp120, and allosteric effects inherent to this IC resulted in higher reactivity with V1V2 apex- specific mAb PG9. Of note, ICs made with novel constructs of stabilized Env trimers (UFO or 1KNC) and V2 mAb 2158 were similarly more reactive with PG9 than their uncomplexed counterparts. PG9 binding was also enhanced upon IC formation with a combination of anti-V2 mAb 2158 and anti-V3 crown mAb 2558, thus offering a strategy to better target the V1V2 apex while occluding the V3 crown. These results provide a compelling rationale for further evaluation of the potential advantages of IC vaccines. This application proposes to evaluate trimeric Env IC vaccines in rabbits and rhesus macaques for the capacity to augment the induction of multifunctional Abs against epitopes in the V1V2 apex and to limit Ab responses to the immunogenic V3 crown. To this end, in Aim 1, we will initially test IC vaccines made of UFO and 1KNC Env constructs and V2 mAb 2158 plus/minus V3 mAb 2558 in rabbits, followed by down selection of the best UFO IC and the best 1KNC IC in rhesus macaque immunogenicity studies. The vaccines will comprise of ICs and UFO Env DNA delivered simultaneously. In Aim 2, we will compare in macaques whether an IC vaccine made with F(ab')2 is as effective as an IC made with intact IgG in directing Ab responses to the V1V2 apex and away from the V3 crown. This aim will assess the importance of Fab vs Fc to IC immunomodulatory activity. mAbs with rhesus Fc will be used in all macaque experiments. Finally, Aim 3 will test whether co-immunization with UFO DNA and IC that elicits Abs with polyfunctional antiviral activities can block or control SHIV infection. At the completion of this study, we will better understand the utility of IC vaccines to modulate Ab response against specific regions of HIV Env. The data will be useful in developing a clinically applicable strategy to produce effective HIV vaccines.